Salt is used in small quantity and is added near the close of the cooking process.
The use of flour or starch in any quantity for the purpose of making the body thick or heavy is properly regarded as an adulteration. This is also true of pulp from a foreign source, like pumpkin or apples.
The density of the ketchup is left usually to the judgment of the chef, who depends upon the appearance as it pours from the ladle. A quick test can be made by weighing, as done for pulp, but in this case each manufacturer must determine his own standard. A ketchup having specific gravity of 1.090 is apt to be thin; a satisfactory consistency is usually about 1.120 to 1.140.
As soon as the cooking is completed, the ketchup is run through a finishing machine to remove all hard particles of tomato, bits of spice, etc., and to give smoothness to the product by breaking it up into very small particles. There are two types of finishers, the shaking sieves and the rubbing machines. The former is suitable for thin ketchup. The resultant product gives the best possible appearance under the microscope, the tissue showing whole cells, little tearing, and the minimum amount of debris and mold filaments. The objections to the sieve are that the capacity is small and the waste is comparatively large. The rubbing finisher needs to be very carefully adjusted, otherwise it forces practically everything through in a very finely comminuted state. The cells of the tissues are torn to shreds, their contents discharged, molds are broken into hundreds of fragments, and a ketchup may be made to have the appearance of being made from poor material. The finishers have large capacity and will work on either light or heavy goods, but like the cyclone, must be handled with judgment, not attempting to force the last ounce through the sieve.
Only new bottles should be used and these should be thoroughly rinsed before using and preferably with hot water. Since new bottles have no tightly adherent particles on the inside, the use of clear water is sufficient, dependence being placed upon the after process to insure sterilization.
The bottling should be done at as high temperature as is practicable, about 165 to 170 degrees F. If the temperature is higher than this, the possibility of burns in handling is increased, and too much space is left in the neck of the bottle after corking, due to shrinkage of the ketchup on cooling, and if much lower, the expansion in processing causes excessive loosening of caps or corks and breakage. Furthermore, when low temperature is used, it requires a very long time to heat the contents of a bottle in pasteurizing. A ketchup is a very poor conductor of heat and the heavier the body, the longer the time that is required.